Due to remarkable improvements in the data recording capacity and processing speed of personal computers (PC), so-called nonlinear editing for performing video editing on a PC is becoming increasingly popular. For capturing video materials to a PC and outputting edited video images, dedicated hardware such as a video capture board, video editing board or the like is additionally inserted in an expansion slot, and data is input/output via such hardware.
Even today, handling analog video and high-end professional signals requires dedicated hardware. However, to handle just data in the DV (digital video) format, which is widely used for consumer devices and low-end professional use, even general inexpensive hardware meeting the standards for a 1394OHCI compliant IEEE1394 interface is sufficient for practical use.
This is because improvements in the CPU capacity of PCs have allowed the practical execution of such processing as video editing without having to use dedicated hardware, and an 1394OHCI compliant IEEE1394 interface is now supported as an interface for a typical video editing software for input/output of data in DV format.
A 1394OHCI compliant IEEE1394 interface is frequently standard not only on desktop PCs but also on notebook PCs, enabling a single notebook PC to be used for everything from inputting/outputting to editing videos in DV format.
If only video material in DV format is to be handled, the process can be completed using a system as described above; however, there are many cases in which analog video images and material in SDI format for professional use need to be handled, and such cases require mutual conversion of formats. To perform mutual data conversion between video material in DV format and analog video images or video material in SDI format, an external unit type DV converter is often concurrently used, such converter converting an input analog video signal or SDI video signal to a DV format in real time and outputting the same as a DV signal, or conversely, converting a video signal in DV format to an analog video signal or SDI video signal and outputting the same.
There are various DV converters, ranging from those for consumer use to those for professional use. Professional use DV converters may require a function called external synchronization (genlock). With DV converters having no such external synchronization function, when a DV signal output from a PC via a 1394OHCI compliant IEEE1394 interface is converted into an analog video signal or SDI video signal, the converted analog video signal or SDI video signal is output at a timing in accordance with the frame frequency of the DV signal output from the PC.
With DV converters having an external synchronization function, a reference signal that serves as a reference for output timing is input via a reference input terminal. When a DV video signal output from a PC 1394OHCI compliant IEEE1394 interface is converted into an analog video signal or SDI video signal, the converted signal, while being buffered, is synchronized with the reference signal and output.
Data transmission between nodes connected on an IEEE1394 bus is either in an asynchronous transfer mode or isochronous transfer mode, and for video and audio transmission, isochronous transfer mode is used. This isochronous transfer mode is also used when a DV video signal is output via a PC 1394OHCI compliant IEEE1394 interface.
When a PC and DV converter exist on an IEEE1394 bus as nodes, and a DV video signal is to be output to the DV converter via a PC 1394OHCI compliant IEEE1394 interface, one of the PC and DV converter serves as a node called a cycle master for managing the transfer cycle, and cycle start packets are output to the IEEE1394 bus at a constant frequency (125 μsec).
The PC 1394OHCI compliant IEEE1394 interface transmits a video signal in DV format in an IEEE1394 defined isochronous transfer packet format each time a cycle start packet output from the cycle master is detected.
Thus, the frame frequency of the DV video signal output from a PC 1394OHCI compliant IEEE1394 interface is synchronized with the frequency of the cycle start packet output from the cycle master. The interval of 125 μsec between cycle start packets output from the cycle master is generated by frequency division at a set ratio from a 24.576 MHz clock source of the node serving as cycle master. Because of differences in clock source for different pieces of hardware, frequency varies. Therefore, because the frame frequency of a DV video signal output from a PC 1394OHCI compliant IEEE1394 interface differs depending on the device, a frame frequency on average will not match the frame frequency of an external reference signal input into a DV converter. As a result, even when buffering is performed at the DV converter, there are problems such as dropped frames in output analog signals or SDI signals in cases where the PC transfer speed is high, and repeated frames in output analog video signals or SDI video signals when the PC transfer speed is low.
The timing of the dropped frames or repeated frames is difficult to predict, and notwithstanding precise editing in frame units using video editing software loaded on a PC, there is the danger that defects such as dropped frames or repeated frames may irregularly occur in the images ultimately output.
The present invention provides a data conversion system configured so that when data output from an information processor is converted into data in a different format in real time, data transfer and converted data output are synchronized, thereby preventing the occurrence of defects in moving image data such as dropped frames or repeated frames.